1. Field of the Invention
The present invention relates to a light emitting diode.
2. Description of the Related Art
The conventional light emitting diode comprises a semiconductor substrate, a light emitting structure on the semiconductor substrate, and two ohmic contact electrodes, wherein the two ohmic contact electrodes are respectively formed on the light emitting structure and on another side of the semiconductor substrate.
In general, the light emitting structure is composed of multilayer group III-V compound semiconductor with aluminum (Al), such as AlGaAs emitting infrared and red light, and AlGaInP emitting yellow-green, yellow, and red light. The light emitting structure emits light in all directions, i.e., it is isotropic. However, the bandgap energy of the substrate is usually less than the energy of the visible light, so the substrate absorbs most of the light emitted by the light emitting structure, which reduces the external quantum efficiency significantly, and reduces the brightness of the light emitting diode accordingly.
A variety of methods are provided to avoid light absorption by the substrate. In addition to the conventional method of interposing the light emitting structure on the substrate between the upper and lower distributed Bragg reflectors (DBRs), U.S. Pat. No. 4,570,172 and U.S. Pat. No. 5,237,581 also disclosed related modified structures. Use of the DBR structure causes the light emitted by the light emitting structure to be reflected when it emits to the substrate, thereby solving the problem of the light absorption by the substrate. However, the DBRs have a high reflective rate only if the incident light enters vertically, and with the increasing incidence angle of the light, the reflective rate is reduced accordingly. Therefore, even when using the DBRs, the external quantum efficiency of the light emitting diode and the ability to increase the brightness are still limited.
U.S. Pat. No. 5,376,580 provides another two methods using wafer bonding technology. The first method involves growing an epitaxial structure of the light emitting diode on a GaAs substrate, and then mounting the epitaxial structure of the light emitting diode to a transparent substrate using wafer bonding technology. The second method involves growing an epitaxial structure of the light emitting device on a GaAs substrate, and then mounting the epitaxial structure of the light emitting diode to a reflective mirror using wafer bonding technology. The above two methods increase the external quantum efficiency of the light emitting diode by removing the GaAs substrate which absorbs the light, wherein the first method uses a transparent substrate to transmit light, and the second method uses a reflective mirror to reflect light. However, the problem of using the transparent substrate in the first method is that the wafer bonding technology should proceed at a high annealing temperature, which results in the redistribution of the dopant among the wafer and reducing the efficiency of the light emitting diode. The problem with using the reflective mirror in the second method is that the reflective surface of the reflective mirror is used to connect directly during the wafer bonding process, which causes the reflective surface to become rough or deteriorated and degrades the reflective surface of the reflective mirror.
U.S. Pat. No. 6,797,987 also provides a light emitting diode using a reflective metal layer by interposing a transparent electrical-conductive oxide layer of an ITO layer between the reflective metal layer and a light emitting structure, so that the reflective metal layer will not function with the light emitting structure during the wafer bonding process. To improve the ohmic contact between the ITO layer and the light emitting structure, the structure of the above patent forms an ohmic contact grid pattern or channels in the ITO layer, or forms an alloy metal mesh between the ITO layer and the light emitting structure. Due to the complicated process of such structure, the manufacturing cost is high. The alloy metal mesh requires a high temperature alloy process and is very difficult to etch the alloy metal being a mesh shape. In addition, a thickness of the alloy metal should be considered: if the alloy metal layer is too thin, then the ohmic contact between the alloy metal and the light emitting structure is poor, while if the alloy metal layer is too thick, the adhesion force after the wafer bonding is weak.
For enhancing the light extraction efficiency of a conventional light emitting diode, U.S. Pat. No. 7,335,924 provides a light emitting diode with a reflective layer. The light emitting diode comprises a light emitting structure, a non-alloy ohmic contact layer sequentially formed on the light emitting structure, a first reflective layer, and a substrate. The major characteristics of the invention lie in a joint effect provided by the non-alloy ohmic contact layer and the first reflective layer so as to solve the problem of light absorption by the substrate. The first reflective layer functions as a reflective mirror and is made of a pure metal or a metal nitride for superior reflectivity. The non-alloy ohmic contact layer is interposed between the light emitting structure and the first reflective layer and the material used for the non-alloy ohmic contact layer can be optically transparent. However, the optically transparent conductive material usually has a higher resistance or a lower light transmittance.
U.S. Patent Publication No. 20050205886 provides a light emitting diode having a transparent penetrating reflective layer. In general, the light transmittance of the transparent penetrating reflective layer is lower than other reflective layer types, resulting in a reduced reflective rate of the light inside the light emitting diode. In addition, the transparent penetrating reflective layer also has a higher coefficient of expansion which is disadvantageous to the manufacture of the light emitting diode, thus reducing the yield rate.
U.S. Patent Publication No. 20040041164 provides a light emitting diode whose major characteristic is that the light emitting diode comprises a light extraction array as a reflective structure, wherein the light extraction array is arranged on a substrate of the light emitting diode. When light is incident to the substrate through the light extraction array, the light can be reflected. However, such reflection requires a total reflection and has a limitation, i.e., the effect is not the same as the direct reflection by metal.
In addition, a paper entitled “Stress behavior of electrodeposited copper films as mechanical supporters for light emitting diodes” published in “LED R&D Lab., LG Electronics Institute of Technology: Electrochimica Acta 52 (2007) 5258-5265” joins a metal and a semiconductor directly. In this method, the inner stress is not released, causing the chip to warp and crack.